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Rock-forming microbes may be suited for life on other planets

By Colin Barras

Building a rock in a hard place

(Image: Michaelis/Ghostdabs, Hamburg)

THEY are, perhaps, the world’s deepest architects. Microbes living beneath the sea floor near methane seeps construct extensive rocky deposits – and then live inside them. The organisms may be representatives of a distinct lifestyle that could be well-suited to conditions on harsh, alien worlds.

Our planet’s crust is home to a surprising diversity of microbes, either living inside crevices in the sediments and rocks below Earth’s surface, or burrowing their way through. But none are quite like the microbes Jeffrey Marlow’s team at the California Institute of Technology in Pasadena have found below the seabed at methane seeps near Oregon.

These microorganisms oxidise methane and generate bicarbonate ions that help them build vast limestone slabs in the soft sediment. What makes them truly unusual, though, is their bizarre lifestyle&colon; they seem to continue living inside cavities within the limestone they create. That life of self-entombment is so unlike anything else seen on Earth that Marlow and his colleagues propose giving it a new name&colon; such microbes should be called autoendoliths.

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“Autoendoliths may be common on and below Earth’s surface, essentially using chemical energy to build physical structures,” says Marlow. We do not know yet how they relate to other bacteria and archaea, but they are likely to be from known evolutionary lineages, says Andreas Teske of the University of North Carolina at Chapel Hill. So, they may be relatives of microbes living around methane seeps that have adopted an unusual lifestyle.

We have known for decades that some microbes can build rocky structures, for instance, stromatolites. But these microbes live on or near the surface of the rocks they make, where they have access to nutrients and light to photosynthesise and metabolise. Autoendoliths, in contrast, inhabit the deep interior of the rocks they build.

So why do they do it? In the case of the Oregon microbes, this lifestyle might provide an advantage in the form of a better energy source – methane-rich fluids bubble up and become more concentrated inside the rocks the autoendoliths create, says Marlow. But other autoendoliths might live deep inside the rock they build to escape harmful conditions nearer the surface, he suggests.

That might have implications for the search for life on Mars and other planets. Astrobiologists already suspected that if life exists on Mars, it probably moved deep underground to escape the intense radiation and extreme temperature swings on the Martian surface. An autoendolithic lifestyle could offer microbes closer to the surface a means to grow their own protective shield from radiation. There is evidence that some surface-dwelling microbes in high-altitude environments on Earth grow mineral coatings to protect themselves from the strong UV radiation there. “It could be seen as an evolutionary advantage in this sort of situation,” says Marlow.

This article appeared in print under the headline “Microbe architects build live-in rocks”